This invention relates generally to constant current sources and speed up circuits for transistor switches and particularly to a novel battery charger utilizing a constant current source and speed up circuit.
Constant current sources are well-known in the art for delivering a fixed RMS value of current under varying conditions of load. Battery chargers are also well-known in the art for supplying current for charging batteries. The invention in one major aspect is directed to a novel constant current source arrangement that is particularly useful in a battery charger.
High speed transistor switching circuits, particularly those used with single-ended power supplies, are limited by the time required to remove base current to the transistor. In double-ended power supplies, i.e., those providing positive and negative operating potentials, the problem of rapidly removing base current is simplified as compared with single-ended supplies where one side of the supply is held at ground potential.
The speed up circuit aspect of the present invention is useful with a single-ended power supply to rapidly remove the base drive to a switch transistor and thereby result in a dramatic enhancement of its switching characteristics. The decrease in switching time enables a significant reduction in heat dissipation in the transistor and permits utilization of a much smaller heat sink. An added advantage of the inventive speed up circuit is that conventional transistor switching circuits may be operated at much higher frequencies.
In many applications, such as in portable television receivers and computers where the batteries comprise a very large part of the overall device cost, it is desirable to use rechargeable battery packs. Rechargeable battery packs generally have nickel cadmium batteries, the useful lives of which are highly dependent upon the manner in which they are recharged. It is common for a manufacturer to guarantee a battery pack for a minimum number of cycles of operation, i.e., discharges and recharges, provided that the batteries are recharged under controlled conditions. Nickel cadmium batteries are best charged either with a low continuous current or with a larger constant current until a certain battery temperature is attained. To do otherwise can have an adverse effect on their useful lives. Consequently, the equipment used to recharge the battery pack needs to be carefully designed. As battery recharging circuits are often built into the powered device, space and weight are at a premium and a high efficiency recharging circuit is desirable.
The present invention is directed to a switching type recharging circuit for supplying a constant RMS current to a battery pack. For the battery pack used, the manufacturer specifies a charging rate of 1.2 amps until a case temperature of 55.degree. C. is attained or a continuous charge of 200 milliamps. The circuit constructed in accordance with one aspect of the invention accomplishes this with very high efficiency, on the order of 84%. When used with the speed up circuit aspect of the invention, an efficiency in excess of 90% is attained. Further, the current supplied to the battery changes less than 1% throughout its charging voltage range.
As will be apparent, the constant current source utilized in the battery charger of the invention will readily find other uses, such as for driving the deflection system in a television receiver or monitor. The constant current source of the invention combines a so-called buck regulator and a voltage sensitive switch with an adjustable hysteresis or delay, to form an oscillator having a duty cycle, the ON time of which is determined by the connected load. Similarly, the speed up circuit will find applicability in many circuit arrangements having a single-ended power supply. In particular, it will find ready application to high voltage systems in television receivers.